1. Field of Invention
This invention relates to hardware for the surgical fastening of human and animal bone, especially when it is desirable to affect high strength fastening.
2. Discussion of Prior Art
In the field of Orthopedic surgery, the fastening together of bone fragments using hardware implanted under the soft tissue is known as "internal fixation". The cause of the bone fragmentation may be surgical osteotomy or traumatic fracture. Certain very serious considerations must be addressed when surgically fastening bone. It is necessary to fasten the bone fragments solidly together until the healing process, including osteogenesis, is completed. It is mandatory to affect a suitably tight fastening, so as to prevent excessive motion between the fragments. The repair must not fail; it must be solid enough to resist failure even with loading imposed by minor accidents. The force required to prevent excessive motion is often great. Such great forces may lead to a problem. Fastener forces can cause bone damage if the force is generally too high or if it is concentrated over too small an area, causing what may be called "point loading". In the process of doing the fastening, it is necessary to cause the least possible disturbance to the bone and associated tissue. The repair must involve minimal stripping of the soft tissue covering the bone.
There are essentially two types of bone, cortical and cancellous. Bones are living, "breathing" tissue. Covering the bone is a soft tissue, the periosteum. This covering contains blood vessels that convey food and oxygen to the bone, and carry waste materials and gasses away from the bone. The outer shell (cortical bone) of a bone is strong, hard and dense with a shear strength approximately ten times that of the inner bone (cancellous). The thickness of cortical bone in usually less than 10 mm while the diameter of the inner bone, cancellous, is commonly 25 to 30 mm. The cortical shell varies in thickness and strength, depending upon where it is located on the bone, and which bone it is. The history of use of the bone and the physical condition of the patient also affect the thickness and strength of the cortex. Cortical bone surrounds a softer, much less dense bone, the cancellous bone. Cancellous bone is somewhat spongy, due to physical design. It is very porous. Cancellous bone has fairly low shear strength and as stated above it is about one tenth as strong in shear as cortical bone. As humans and animals age, the percentage of cortical bone decreases while the percentage of cancellous bone increases. With all of the above considerations, bone fastening requires some very special techniques.
There are several methods used to fasten bone fragments together. Until this century, man had only one method for repairing bone fractures; closed reduction. The bones were anatomically re-aligned (fracture reduced) and the muscles and a splint (later a cast) held the bones in alignment and immobile until the fracture healed. Open fractures were virtually always fatal. In the last one hundred years man has been able to repair both open (compound) and seriously comminuted (splintered) closed fractures. In the last 50 years, surgeons use wire, smooth pins, rods and threaded fasteners, both with and without plating hardware. Because of specific loading limitations as well as other considerations, hardware such as washers and plates are used with fasteners to spread the load over a greater area. Plates may also serve to couple together an array of fasteners.
Wires are adequate for fracture fixation in a small percentage of bone fractures, preferred mostly when the fracture line is linear to the bone. Such fractures may occur when the hoop strength of the bone is exceeded, such as is generated when an object is driven into the medullary canal with so much force that it splits the bone outward. To fasten such a fracture, the wire is wound several times around the fractured bone, holding the fragments together, and the wire ends are twisted to secure the fastening. U.S. Pat. No. 5,697,934 describes a wire fastener. A variation on this wire technique employs a cable and special clip. The cable is tightened with a special tool, and the clip is crimped over the cable ends to secure them. Besides the limited strength of wire fixation, a further problem with this general type of fastening, called "cerclage wiring", is that it requires more surgical invasion and periosteal stripping than other methods of fastening. To expose naked bone the periosteum must be stripped off the bony surface. It is widely accepted that stripping soft tissue from bone must be kept to a minimum.
Smooth pins are sometimes used for fastening bones, but this is typically for low strength requirements and temporary fixation.
Rods, also called IM Nails, are used inside the bone, to restore shape. Such rods are variably locked (cross-fastened) to the bone with additional bone fastener hardware. Limitations with IM Nailing are that the installation requires additional major surgery, the rods prevent the bone from receiving strengthening exercise, and this "stress shielding" leads to bone weakening. Removal of the rods requires major surgery.
Threaded fasteners (bone screws) and washers are commonly used for fastening bone fragments together. There are many such fasteners on the market. The firms AO and Zimmer offer a number of threaded fasteners. Such fasteners work well in limited stress situations. Bone screws look rather like wood screws. The anchoring force of such screws comes from the shear strength and the hoop strength of the bone into which the screw is driven. The strength limitation encountered in fastening with bone screws is at least in part a function of physical properties of bones. As shown above, a large portion of any bone is cancellous bone, and cancellous bone is porous. The high-density portion of bone, the cortical bone, is of variable strength; therefore in many cases bone does not provide very good anchoring for threaded fasteners. Because of this, there are special screws with especially large threads (adding to the area) to catch more of this weak bone. (One such bone screw is described in U.S. Pat. No. 5,129,901) Such fasteners also rely upon the shear strength and hoop strength of the bone to anchor the threads of the screws. With advancing age, the reduction of cortical bone mass with the increase of cancellous bone mass reduces the ability of the patient's bone to anchor threaded fasteners. When the forces upon the bone (and thus the fastener) are great, the hoop strength and/or shear strength of the bone is exceeded, and the fastening fails (it pulls out allowing the bone fragments to separate)--catastrophically. A further disadvantage of bone screws which anchor into bone is the high and indeterminate torque of installation. A surgeon is never certain that the torque he is providing is tightening the bone fragments together (closing the fracture) or merely overcoming the torque required to drive the fastener further into the cortical bone.
Another fastener for bones is a bolt and nut. In the past, surgeons wanting high fastening strength had to use a bolt and a nut. The Webb Bolt, the Barr Bolt and the Zimmer Tibia Bolt exemplify this type of fastener. Because the available bolt length would not be exactly what was required, it was necessary to cut off the excess length of the bolt using a pinching action cutter. The cutting action left behind a sharp stump. Such cutting has some obvious disadvantages. 1) The cut destroys the thread profile, making disassembly and re-assembly of the bolt and nut difficult or impossible. 2) The cutting action imparts a force couple to the bolt, transmitting unacceptable forces to the surrounding bone. 3) The sharp edge remaining from the pinching off is harsh on the soft tissues, causing pain and trauma. 4) If this sharp edge is filed off, the debris is unacceptable to the body. A further disadvantage of the nut and bolt fastener is that the threads of the bolt pass unprotected through the cortical bone, and these unprotected threads are free to dig into the cortical bone, creating stress risers that can lead to failure. A further disadvantage of the nut and bolt fastener is that the washers were flat and fit flat against the bolt head and the nut. The washers could not tilt to spread the load evenly. Another disadvantage of this type of bolt and nut is that it was not cannulated. It could not be installed over a guide wire. Yet another disadvantage of this nut and bolt is found in the installation tooling. To afford truly minimally invasive installation, it is necessary to have complete control of the fastener until the surgeon is completely finished with the installation. The nut and bolt fastener could not be rigidly coupled to the installation tool. A surgeon could not readily insert the fastener accurately into a deep hole, and then direct it unerringly laterally and/or angularly. At the time of removal, the surgeon could not easily grasp the bolt or nut down deep in soft tissue and pull it from the bone.